The invention relates to an echo canceller for connection between a first and a second one-way transmission path having opposite signal directions and being connected to the four-wire side of a four-wire-to-two-wire coupling circuit, having for its purpose to cancel an echo signal occurring in the second path in response to the signal in the first path, the echo canceller including:
a controllable digital signal processing arrangement, connected to the first path and having an input for receiving a control signal, for providing a synthetic echo signal in response to the signal in the first path and the digital control signal, PA1 a difference circuit for producing a residual signal resulting from the difference between the signal in the second path and the synthetic echo signal produced by the controllable signal processing arrangement, and PA1 an analog-to-digital converter for converting the residual signal into the digital control signal for application thereof to the controllable digital signal processing arrangement.
An echo canceller is used in, for example, a data transmission modem whose transmitter and receiver are interconnected by means of the coupling circuit, usually referred to as a hybrid junction, in a manner such that the mode has a two-wire access to the exterior. It is known that when a connection is provided between the two-wire access of two modems, an echo signal may be produced in the receive path of a modem, which echo signal is formed by a portion of the signal from the transmit path of the same modem and is due to imperfections of the hybrid junction thereof and/or reflections of the transmit signal in the connection. An echo canceller is used to automatically canel this echo signal appearing in the receive path. The advantage of this echo cancellation is that it enables a simultaneous transmission between two modems which are interconnected by way of their two-wire access.
The structure of the above-described echocanceller is well-known and disclosed in, for example, Applicants' French published Patent Applications Nos. 2,358,790 and 2,377,734, which relate to baseband and voice-band data transmission, respectively. The controllable digital signal processing arrangement is generally an adaptive digital filter or an assembly of digital filters coupled to the transmit path of the modem and comprising a control circuit for the coefficients of this filter or of this filter assembly. In order to simplify the terminology, the controllable digital signal processing arrangement will be simply denoted digital filter hereinafter. This filter supplies a digital signal which is designated the synthetic echo signal and is converted into the analog form before being applied to an input of the difference circuit, the other input of which receives the signal which appears in the receive path of the modem and which includes the echo signal to be cancelled. The signal produced by the difference circuit is denoted the residual signal hereinafter. Samples of this residual signal are applied to an analog-to-digital converter which produces the digital version of the residual signal used as a control signal for the control circuit for the filter coefficients. The circuitry described in, for example, the above-mentioned Patent Applications are used in this control circuit and makes it possible to adjust the coefficients iteratively so that these coefficients converge to values which minimize the mean square error of the remaining echo signal present in the residual signal and resulting from the difference between the real echo signal and the synthetic echo signal supplied by the filter. This convergence, resulting in, the cancellation of or substantial decrease of the echo signal, must be realized in the presence of the received useful data signal originating from the remote modem. This useful data signal is superposed on the echo signal remaining in the residual signal and its level with respect to the level of the remaining echo signal becomes increasingly large during the convergence. In certain cases, this may cause a problem which results from the analog-to-digital conversion of the residual signal, and which may result in stopping of convergence at a comparatively high level of the remaining echo signal.
This problem is encountered, in particular, in the case of homochronous data transmission if one wants to employ a less accurate analog-to-digital converter in order to reduce cost. It is known that in a homochronous data transmission system, the data are transmitted in synchronism with a local clock synchronized to the received data. In the echo canceller, this local clock is also used to time the sampling pulses for the residual signal. From this it follows that the sampling instants of the residual signal have a fixed position in time with respect to the received data signal. If, for example, one wants to use a very simple analog-to-digital converter producing a digital signal having one bit indicating only the polarity of the residual signal, the value of this digital signal is only of significance for the polarity of the remaining echo signal if, at the sampling instants, the amplitude of this remaining echo signal is greater than the amplitude of the received data signal. In a homochronous transmission system, the opposite situation may permanently prevail in which no change of the filter coefficients, tending to a reduction of the remaining echo signal, will occur, this remaining echo signal staying at a high value in the order of magnitude of the received data signal. The same problem is encountered, although to a lesser extent, if a somewhat more accurate 2- or 3-bit converter is used, for the remaining echo stays at a value of the order of magnitude of the smallest quantization step. A solution which allows use of a converter producing a digital one-bit signal and which yet allows convergence of the coefficients to a very low value of the remaining echo signal is disclosed in U.S. patent application Ser. No. 128,418, filed on Mar. 10, 1980, now U.S. Pat. No. 4,334,128. This solution consists of combining the residual signal with an auxiliary signal having no correlation with this residual signal and employing this combined signal, after conversion into a digital one-bit signal, for the control of the coefficients.